The present invention relates to a pressure detection switch, which is actuated by detecting the switchover setting pressure, and to a hydraulic pressure control system to cut off the power circuit of the motor for driving the pump by the operation of the above pressure detection switch when the accumulator pressure reaches the predetermined pressure.
In a hydraulic pressure control system using hydraulic pressure such as hydraulic braking system, the pump and accumulator are often used as pressure source. In such case, the operating fluid as discharged from the pump is accumulated in the accumulator at the predetermined pressure, and the operating fluid at the pressure necessary for operating hydraulic equipment such as hydraulic booster can be reliably and stably supplied to the hydraulic equipment. Also, the accumulator is provided with such function that it can operate the hydraulic equipment for a certain period of time even when pump is in trouble.
In this way, the accumulator can accumulate operating fluid of the predetermined pressure, whereas the continuous operation of the pump after the predetermined pressure is reached in the accumulator may result in useless waste of fuel and may shorten the service life of the pump.
In the conventional system, therefore, the hydraulic pressure control system is adopted, which stops the driving of the pump when accumulator reaches the preset pressure as shown in FIG. 3.
In FIG. 3, the hydraulic pressure control system is provided with the hydraulic pressure source, which comprises a pump 2 to supply the operating fluid to the hydraulic equipment 1 such as hydraulic booster, an accumulator 3 and a check valve 4 to allow only the flow of the operating fluid from the pump 2 to the accumulator 3. Further, the system is provided with a pressure detection switch 7', which is mounted on a power circuit 6 of the motor 5 to drive the pump 2. This pressure detection switch 7' detects that the pressure of the accumulator 3 reached the preset pressure and cuts off the power circuit 6. At the same time, the switch detects that the pressure of the accumulator 3 is reduced to lower than the preset pressure, and it controls the driving of the pump 2 by connecting the power circuit 6. Namely, the switchover setting pressure of this pressure detection switch 7' is set to the preset pressure of the accumulator 3.
It is possible by such hydraulic pressure control system to drive the pump 2 efficiently, and this contributes to the saving of fuel and to the improvement of the durability of the pump 2.
As it is evident from FIG. 4, the pressure detection switch 7' used in this hydraulic pressure control system comprises a pressure fluid inlet 8, into which the pressure fluid of the accumulator 3 is introduced, a piston 10, which receives the hydraulic pressure inside the pressure fluid inlet 8 on one end and also permanently receives the predetermined spring force by a compression coil spring 9' on the other end, a pushing unit 11 to be operated by the movement of the piston 10, and a permanently closed switch unit 12, which is turned off by the operation of this pushing unit 11 and cuts off the power circuit 6 of the motor 5.
Therefore, when the pressure fluid at the pressure fluid inlet 8 reaches the preset pressure to overcome the spring force of the spring 9', the piston 10 is moved against the spring force and moves the pushing unit 11 to the same direction. By the movement of the pushing unit 11, the switch 12 is turned off, and power circuit 6 is cut off. In this case, the spring force of the compression spring 9' is set to a constant value regardless of ambient temperature, and the switchover setting pressure of the pressure detection switch 7' is kept at a constant level regardless of ambient temperature.
However, if the switchover setting pressure of the pressure detection switch 7' is kept at a constant level regardless of ambient temperature, on-off control of the pump 2 is also set to a constant level regardless of ambient temperature. Thus, when ambient temperature is increased and internal pressure of the accumulator 3 is also increased with the internal pressure reaching a certain value, i.e. the internal pressure of the accumulator reaches a certain value with the increase of ambient temperature, the power circuit 6 is turned off by the pressure detection switch 7'. Accordingly, this leads to the problem that total quantity of pressure fluid accumulated in the accumulator 3 is reduced.